1. Field of the Invention
The present invention relates generally to a control device for a wound field synchronous machine which generates magnetic fields by field windings of a rotor and, more particularly, to a control device for a wound field synchronous machine which generates magnetic fields by a combination of field windings of a rotor and permanent magnets.
2. Description of the Background Art
Conventionally, a vector control method is used as a high-efficiency, high-power-factor control method for controlling a wound field synchronous machine (which will occasionally be referred to simply as the synchronous machine hereinafter) in such a manner that armature flux linkage and armature current are perpendicular to each other as shown in a paper titled “High-performance Variable Speed Control of Synchronous Machine” written by Hiroshi Osawa, et al. published in The Transactions of the Institute of Electrical Engineers of Japan on Industry Applications, Volume 107-D, No. 2, pages 175–182, February 1987, for instance. A synchronous machine using permanent magnets together with field windings, however, can not employ the conventional vector control method as it is necessary to take into consideration the influence of magnetic fluxes produced by the permanent magnets.
There has been a conventional approach to the solution of the aforementioned problem as shown in Japanese Laid-open Patent Application No. 1996-331900, for example, in which a control device determines, from a torque command given to the control device and turning speed of a synchronous machine, an armature current command value for a field flux direction (d-axis), an armature current command value for a direction (q-axis) perpendicular to the d-axis and a field flux command value referring to a table such that maximum efficiency is achieved under current operating conditions of the synchronous machine. Individual data values stored in such a table are calculated by using characteristic parameters of the synchronous machine to be controlled. A similar approach to the problem is also shown in Japanese Laid-open Patent Application No. 1999-313498, in which a field current command is determined instead of a field flux command from a table.
The conventional synchronous machine control device structured as mentioned above determines a d-axis current, a q-axis current, and the values of field-generating commands, such as the field flux command or the field current command, referring to the table. While the individual data values stored in the table are calculated by using the characteristic parameters of the synchronous machine to be controlled, this process is complicated and necessitates repetitive calculations. For this reason, these data values are calculated off-line and stored in the table in advance. The conventional control device makes it possible to perform maximum torque controlled operation with any armature current, for instance, to obtain a maximum torque in response to the torque command given by generating individual current commands referring to the table produced as explained above.
In actual production lines, however, it is fairly difficult to manufacture synchronous machines having perfectly the same characteristic parameters, which include inductance and permanent magnet fluxes, for instance, even if the synchronous machines are of the same model. Accordingly, there inevitably occur differences in the values of the characteristic parameters among the individual synchronous machines due to variations in their production. According to the aforementioned conventional control method, it is necessary to produce data values to be stored in the table for the individual synchronous machines when the values of the characteristic parameters differ from one synchronous machine to another, and this entails great deal of work load. Additionally, the synchronous machine and the control device are usually shipped separately, and not as a preadjusted set, and the synchronous machine and the control device are joined and run together at a customer's site, making it necessary to store the data values of the table appropriate for each particular synchronous machine in the control device.
According to the conventional control method, it is necessary to store d-axis currents, q-axis currents and field flux command values for different combinations of torque commands and machine turning speeds in the table. This means that an extraordinarily large amount of data must be registered and this procedure can not be easily done and renders the vector control method impracticable.